The 10 round mag I have that says P99 on the floorplate is factory. I also have some 10 round mags that look like the ones in this post: http://www.waltherforums.com/showthread.php?t=10851
which seem to be for the P88C. The floorplates are a different shape than the one marked P99, everything else appears the same. The metal part of the magazine both have patent #5386657. The plastic floorplate on the P99 marked one has the # 2628180, and the other plastic floorplates have # 2618249 on them. Are they interchangeable, or specific for the P99 and P88C?

the p99 and p88c magazines are basically the same. start with the p88c magazine tube. straighten out the side dimples at the bottom. put in the new shape follower and spring. replace the flat metal base plate with the plastic one. now you have yourself a p99 magazine.

it is likely that,for the 10 round magazines,walther used the same tubes. it is also likely that they use the same springs and followers.

on the standard capacity magazines the only thing that stops you from using the p99 magazine is the base plate. switching the plastic base plate with a flat metal one from a beretta 92 or cz75 or hi-power will allow you to use the standard p99 (or sw99) magazine. you even gain one or two rounds.

i have been using sw99 magazines in my p88c for a couple of years without any problems.

if the 10 round p99 magazine will seat in the p88c,i don't think you will have any problems.

i see that you are located in new york. i think there are restrictions on standard capacity magazines there. is that correct ?

there are pre-ban magazines out there for the p99. i think any p99 16 round non le marked magazine would be pre-ban. i think the change to 15 rounds happened during the ban.

then there is the option of converting the beretta 92 magazine for use in the p99.

Thanks for the input lechiffre. I do have pre-ban 16 round mags for the P99. And pre-ban hi caps for the P88C. I just wanted to make sure I used the right 10 round mags in the right gun, and your help is much appreciated.